1. Field of the Invention
This invention relates to a cathode material for manufacturing a cathode of a rechargeable battery, and more particularly to a cathode material including a first compound and a second compound for manufacturing a cathode of a rechargeable battery. This invention also relates to a cathode and a rechargeable battery with a cathode employing the cathode material.
2. Description of the Related Art
Along with diversified development of electronic products, there is increasing need for portable power supplies. For example, electronic consumer products, medical devices, motorcycles, automobiles, and power tools and the like require a portable power supply for power source. For current portable power supplies, rechargeable batteries are relatively popular. Since lithium rechargeable batteries have a high ratio of volume to capacity, pollution-free and recyclable charge/discharge properties, and no memory effect, it has a great development potential in the future.
Additionally, a cathode material used for manufacture of the cathode plays an important role in the performance of the rechargeable battery. Among the known cathode materials, since lithium ferrous phosphate compounds and the related compounds having similar properties to LiFePO4 compounds, such as LiMPO4, in which M represents transition metal elements, e.g., manganese (Mn), cobalt (Co), and nickel (Ni), are environmentally benign, relatively stable and abundant, and have relatively good electrochemical properties, e.g., high specific capacity, good charge/discharge cycling performance, and good thermostability, they have been evaluated to be the cathode material with greatest development potential.
However, at present, there is a difference between practical and theoretical electrochemical properties of known LiFePO4 compounds and the related compounds. For example, the theoretical specific capacity of LiFePO4 compounds and the related compounds is about 170 mAh/g, whereas the LiFePO4 compounds disclosed in U.S. Pat. No. 5,910,382 have a specific capacity of about 95 mAh/g, which is far below the theoretical specific capacity. In order to improve the capacity property of the LiFePO4 compounds, it has been proposed to add other elements to the LiFePO4 compounds having one of olivine and NASICON structures so as to increase the capacity property of the LiFePO4 compounds, see U.S. Pat. Nos. 6,716,372 and 6,815,122. However, since the elements used for substituting iron are not easily available, production cost is relatively high.
In addition, U.S. Pat. No. 6,632,566 (hereinafter referred to as the '566 patent) discloses increase in the specific surface of the LiFePO4 compound powders in favor of diffusion of lithium ions in the powders, thereby enhancing capacity of a cathode material made from the LiFePO4 compound powders. Particularly, the cathode material described in the '566 patent is produced by sintering the LiFePO4 compound powders at a suitable temperature in such a manner that the cathode material thus formed is composed of separate single-phase crystalline particles having a grain size not larger than 10 μm. However, the LiFePO4 compound powders included in the cathode material have a relatively large particle size, the rechargeable battery with the cathode material of the '566 patent isn't good enough at the charge/discharge properties. Besides, since the charge/discharge rate of the rechargeable battery with the cathode material of the '566 patent is about C/37, which is calculated based on data shown in the Examples of the '566 patent, such charge/discharge rate is too low for practical application and needs to be improved.
Since conductivity and lithium ion diffusion rate of lithium ferrous phosphate are still relatively low, the specific capacity will decrease at a high charge/discharge rate so that use of such lithium ferrous phosphate in high current density applications is restricted. In order to improve the conductivity of such lithium ferrous phosphate, it has been proposed to further reduce particle size of lithium ferrous phosphate or add conductive material to lithium ferrous phosphate. For example, Japanese Patent Publication No. 2003-323892 (hereinafter referred to as the '892 publication) discloses a method for making a cathode material, which involves mixing lithium ferrous phosphate powders, conductive metal particles and a polar solvent in a hermetic container at a temperature ranging from 100° C. to 250° C. Since the method of the '892 publication is required to be conducted at a relatively high temperature and pressure, and since the activity of the conductive metal particles is relatively high, it is difficult to control operating conditions in the method of the '892 publication and hence, production cost of the method is relatively expensive.
Another approach to mix conductive material with the lithium ferrous phosphate involves addition of organic material to a reactant mixture containing lithium ferrous phosphate. For example, in a solid-phase mixing process, lithium salt, iron salt, and phosphate are formed into lithium ferrous phosphate powders, while the organic material is pyrolyzed to produce alkane and alkene gases, and conductive carbonaceous materials that are scattered in the lithium ferrous phosphate powders. However, the pyrolyzed products, i.e., alkane and alkene gases, cause environmental pollution and raise environmental concerns. Accordingly, this method is not recommended.
The grandparent U.S. patent application Ser. No. 11/222,569 (hereinafter referred to as the '569 application) discloses a method for making a lithium mixed metal compound, such as the lithium ferrous phosphate compounds having an olivine structure. The method disclosed in the '569 application includes steps of preparing a reactant mixture that comprises ion sources of the lithium ferrous phosphate compounds, and exposing the reactant mixture to a non-oxidizing atmosphere in the presence of suspended carbon particles. The lithium ferrous phosphate compounds thus made are in powder form and have relatively small particle size and carbon are even distributed.
The parent U.S. patent application Ser. No. 11/510,096 (hereinafter referred to as the '096 application) discloses a cathode material that has one of olivine and NASICON structures and that includes micrometer-sized secondary particles, each of which is composed of crystalline nanometer-sized primary particles of a metal compound. The cathode material may be produced by modifying the method called for in the '569 application. Compared with the conventional cathode material, the cathode material of '096 application has an improved specific surface area and capacity. In addition, the cathode material of '096 application is compatible with various binders, particularly aqueous binders, and aqueous solvents, particularly deionized water, and can be easily coated on and firmly adhered to the electrode plate.
However, there is still a need in the art to provide a cathode material with relatively small particle size and relatively excellent conductivity that can satisfy electrochemical properties and that can be produced by an economical and environmental friendly method.